Fix emphasis tests with multiple underscores wrongly converted

When underscores are used inside words, they must not be converted to
elements. This change fixes the case where multiple underscores are
used, e.g. `this___isa___test`.

lib/kramdown/parser/kramdown/emphasis.rb

@@ -22,7 +22,7 @@ def parse_emphasis
         element = (result.length == 2 ? :strong : :em)
         type = result[0..0]
 
-        if (type == '_' && @src.pre_match =~ /[[:alpha:]]-?[[:alpha:]]*\z/) || @src.check(/\s/) ||
+        if (type == '_' && @src.pre_match =~ /[[:alpha:]]-?[[:alpha:]]*_*\z/) || @src.check(/\s/) ||
             @tree.type == element || @stack.any? {|el, _| el.type == element }
           add_text(result)
           return